PDS_VERSION_ID = PDS3
RECORD_TYPE = STREAM
OBJECT = TEXT
NOTE = "Known errors and/or anomalies in the volumes"
PUBLICATION_DATE = 2007-06-29
END_OBJECT = TEXT
END
ERRORS AND/OR ANOMALIES IN THE CURRENT VOLUME
Volume CORADR_0090, Version 01
------------------------------
1. The Cassini Radar Transition file (EXTRAS/CRT_0090_V01.TAB) contains
no information about ScanStart and ScanEnd transitions.
2. The uncompressed LBDR product has an attached label.
Normally, a file that has been compressed with ZIP would have been
generated without an attached label.
3. In the volume index table (INDEX/INDEX.TAB), double quotes enclose
all the date/time values. Normally, PDS date/time values are not
quoted, but quoting makes parsing easier for some applications.
4. The HTML documents in the DOCUMENT directory contain HTML character
codes that, while all-ASCII, are not easily interpretable by someone
who is reading the HTML documents as text documents. For example,
"α" represents the lower-case Greek character "alpha" and is
rendered as such in a web browser. Equivalent and more legible character
codes (e.g., "alpha;") are available as of the HTML 4.0 specification
but cannot be used here, as PDS requires HTML documents to comply to the
HTML 3.2 specification.
5. The active mode science data segment fields in the SBDR and LBDR products
are invalid and flagged accordingly. Distant satellite observations
do not generate burst by burst active mode results. Instead, the science
results for distant satellite observations are summarized in the AAREADME.TXT
file in the top level directory of this volume.
6. The science data quality flag only specifies the validity of the active
and passive boresight geometry when the corresponding active or passive data
valid bit is zero (valid). For example, if bit 1 is set (invalid) then all
the active boresight geometry is invalid even though bits 7 and 8 may be
zeroed.
7. The user should be aware that the antenna main beam usually covers all
or a significant fraction of the target body during distant satellite
observations. The boresight geometry fields are of limited value since
they apply only to the center of the antenna beam. In some cases
spacecraft pointing control puts the boresight off of the target and these
fields are then invalid.
8. The instrument is operating in a special mode for this data take.
Multiple bursts in flight are used to accomodate the very high range
to the target. This slightly alters the meaning of the active mode
geometry fields in the science data segment of the SBDR and LBDR files.
9. Antenna temperature, brightness temperature, and receiver temperature are
defined in Janssen, M. A., "An Introduction to the Passive Microwave Remote
Sensing of Atmospheres," Chapter 1 in Atmospheric Remote Sensing by
Microwave Radiometry, (M. Janssen, ed.), pp. 1-35, Wiley & Sons,
New York (1993). The archived value in the SBDR and LBDR files gives
uncalibrated antenna temperature in units of Kelvin.
The best current algorithm for correcting the archived antenna temperatures is
Ta_corrected = Ta_archive * ( 0.920 - 0.0041*( t - 1.90 ) )
where t = time in years and fractional years since 2004.0
(0 UTC on 1 Jan 2004)
This algorithm is based on the radioastronomical flux scale at 2-cm wavelength
by direct comparison of distant (unresolved) Titan measurements by the Cassini
radiometer with VLA measurements of Titan reported by Butler and Gurwell,
2004. This algorithm will improve with time as more distant Titan
measurements are obtained and as more radioastronomical comparison sources are
included.
Butler, B. J., Gurwell, M. A. 2004. Radio Wavelength Observations of Titan
with the VLA. Bull. Am. Astron. Soc. 36, 6.04.
This algorithm applies also to all preceeding volumes.
10. Ideally the calibrated antenna temperature is referenced to cold sky at
2.7 K, although no guarantee is made that this zero-level accounting has been
correctly made. Also, the antenna temperature is not necessarily the
brightness temperature seen in the main beam; e.g., there will be an offset
if the sidelobes happen to fall on other than cold sky. The antenna pattern
has low but significant gain at large angles from the boresight which can
introduce thermal power from other targets besides the intended target in
the main lobe. In particular, there is an offset to be expected and
accounted for when an extended source like Titan or Saturn is observed
from a close distance. System gain is the quantity that multiplies the
raw sky counts to convert to the uncalibrated Kelvin scale.
Receiver_temp is the receiver noise temperature Tr (comparison made at
internal reference switch). The receiver temperature plus the antenna
temperature is equal to the total signal (raw counts times system gain).
11. ant_temp_std is a measure of the rms uncertainty of Ta, and is only an
estimation. It is obtained as the standard deviation of Ta for three points,
Ta(I-1), Ta(I), and Ta(I+1), and is a useful measure that identifies
questionable data. For example, it gets large when the beam is sweeping
across a brightness discontinuity.
ERRORS AND/OR ANOMALIES IN PREVIOUS VOLUMES
Not Applicable